Many attempts have been made to design mechanisms for the six degree of freedom (6-DOF) control of a moveable platform. The applications have been diverse, from machine tool positioning to realistic force-reflecting master hand controllers.
Stewart Platform
One of the oldest mechanisms for platform control is the Stewart platform (Stewart D., 1965, "A Platform with Six Degrees of Freedom", Proceedings of The Institution of Mechanical Engineers, London, England, Vol 180, pp 371-386). It is in wide use for heavy-lift applications such as aircraft simulators. Six struts are used to join a suspended platform to its base; they lie along the edges of an octahedron, to give a highly symmetric arrangement. Each strut contains a prismatic (sliding) actuator, and is connected to the base by a universal joint (two orthogonal intersecting revolute joints) and to the platform by a spherical joint (three orthogonal intersecting revolute joints).
Crossed-strut Stewart platform
Staughton and Arai (Stoughton R. & Arai T., 1993, "A Modified Stewart Platform Manipulator with Improved Dexterity", IEEE International Conference on Robotics and Automation) suggest an improvement over the Stewart platform, created by allowing the struts to cross over one another, and attach at more distant points on the base and platform. Three struts are mounted with their end points on an outer circle, with the intervening three on an inner circle. The longer struts are closer to horizontal, giving improved force capacity in the plane parallel to the base and increased torque about a normal to the base.
Arai et al (Arai T., Stoughton R., Homma K., Adichi H., Nakamura T. & Nakashima K. 1991, "Development of a Parallel Link Manipulator", IEEE International Conference on Robotics and Automation) describe a modified Stewart platform with six struts crossed over to give near-isotropic force and moments, with some workspace limitations. The application is an underground excavation task.
Six-Arm Design, with Serial Linkages in Each Arm.
Pierrot et al (Pierrot F., Dauchez P. & Fournier A., 1991, "HEXA: A Fast Six-DOF Fully-Parallel Robot", IEEE International Conference on Robotics and Automation) describe a design with six independent articulated arms supporting a plate hanging below a base. The arms are arranged in pairs in order to simplify the arrangement of actuators; each arm is driven by a rotary actuator at the shoulder that alters the arm's pitch.
Triple Arm Designs, with Serial Linkages in Each Arm.
Cleary and Brooks (Cleary, K. and Brooks, T., 1993, "Kinematic Analysis of a Novel 6-DOF Parallel Manipulator", IEEE International Conference on Robotics and Automation, Atlanta, Ga., pp. 708-713) present a 6-DOF device combining three 2-DOF linkages. Driven by a pair of motors through a differential gear, each linkage can be rotated in pitch and roll to steer the suspended platform.
Triple Arm Designs, with Five-Bar Linkages in Each Arm.
Iwata (Iwata H., 1990, "Artificial Reality with Force-feedback: Development of Desktop Virtual Space with Compact Master Manipulator", SIGGRAPH, Dallas, Tex., Vol. 24, No. 4, pp. 165-170) built a 9-DOF device that provides 6-DOF motion to the hand and 1-DOF motion to each of three fingers. The main platform motion is provided by three pantographs (five-bar linkages with zero length base member) connected by universal joints (U-joints) to the corners of the triangular platform. Each pantograph is driven by two rotary actuators at the base.
Long and Collins (Long G. & Collings C., "A Pantograph Linkage Parallel Platform Master Hand Controller for Force-Reflection", IEEE International Conference on Robotics and Automation, Nice, France, May 1992, p 390) report a 6-DOF joystick with three parallel pantograph linkages.
Mimura and Funahashi (Mimura, N. and Funahashi, Y., 1995, "A new analytical method applying 6 DOF parallel link manipulator for evaluating motion sensation", IEEE International Conference on Robotics and Automation, p 227) describe a similar mechanism, with three five-bar mechanisms (with no-zero length base links) in place of the pantographs. Double Arm Design, with Serial Linkages in Each Arm.
Iwata (Iwata H., 1993, "Pen-based Haptic Virtual Environment", IEEE International Symposium Conference on Robotics and Automation) presented a 6-DOF haptic pen positioned by two 3-DOF serial manipulators. One end of the pen is connected to one of the manipulators by a universal joint (two orthogonal recolute joints with interesecting axes), while the other end is connected to the second manipulator through a screw mechanism and a universal joint. If the two robots move such that the distance between the universal joint centers is constant, the pen moves with five degrees of freedom: three in translation and two in rotation (pitch and roll about the pen). Changes in the distance between the two universal joint centers result in a yawing motion of the pen due to the screw mechanism.
Rigid Strut Designs, with Base-Sliding Supports
Behi (Behi F., 1988, "Kinematic Analysis for a Six-Degree-of-Freedom 3-PRPS Parallel Mechanism", IEEE Journal on Robotics and Automation) describes a robot manipulator with three rigid struts. The struts are attached by spherical joints to the corners of a triangular platform, and at their other ends to sliders which run along the sides of a triangular base. Each of the three sliders carries a rotary actuator, which controls the pitch of the strut attached to that slider.
Hudgens and Tesar (Hudgens J. and Tesar D., 1992, "Analysis of a Fully-Parallel Six Degree-of-Freedom Micromanipulator", IEEE International Conference on Advanced Robotics) present a device with six rigid struts attached by spherical joints to both the platform and the base. The joints are equidistant from the centers of the platform and the base; the struts are arranged in three pairs, the members of each pair being parallel and a short distance apart. The base joints can be pulled or pushed (by actuators connected to eccentric drives) along the circumference of the base circle, thereby giving rise to small movement in the platform.
A device presented by Mouly and Merlet (Mouly N. & Merlet J., 1992, "Singular Configurations and Direct Kinematics of a New Parallel Manipulator", IEEE International Conference on Robotics and Automation, Nice, France, pp 338-343) also has six rigid struts; these are connected by spherical joints at one of their ends the corners of a triangular platform, and at the other end (by U-joints) to moveable supports near the corners of a triangular base. The supports move vertically, so the end of each strut can rise and fall relative to the surface of the base.